patch 9.1.1725: Wayland code can be improved

Problem:  Wayland code can be improved
Solution: Refactor Wayland Clipboard code (Foxe Chen)

This refactor makes the Wayland codebase less convoluted:
- Move clipboard code in wayland.c to clipboard.c
- Use C99 bool type
- Properly poll the Wayland display file descriptor
- Instead of checking if the data source is not NULL in order to
  determine if a selection event comes from us, use a special mime type to
  identify selection events coming from ourselves. The problem with the
  previous approach is that race conditions may occur.
- Put the focus stealing code under a new feature "wayland_focus_steal"
- Use ELAPSED_* macros instead of gettimeofday()
- Pass tests
- Reimplement commented out code
- Update docs
- Make Wayland clipboard behaviour more in line with X11 when connection is lost
- add missing malloc checks and possible memory leaks + refactored some
  tests.

closes: #18139

Signed-off-by: Foxe Chen <chen.foxe@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/various.txt

@@ -1,4 +1,4 @@
-*various.txt*   For Vim version 9.1.  Last change: 2025 Aug 18
+*various.txt*   For Vim version 9.1.  Last change: 2025 Sep 01
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -528,6 +528,8 @@ T  *+vreplace*		|gR| and |gr|
    *+vtp*		on MS-Windows console: support for 'termguicolors'
 N  *+wayland*		Unix only: support for the Wayland protocol.
 N  *+wayland_clipboard*	Unix only: support for Wayland selections/clipboard.
+N  *+wayland_focus_steal* Unix only: support for Wayland clipboard on
+			compositors without a data control protocol.
 T  *+wildignore*	'wildignore'  Always enabled since 9.0.0278
 T  *+wildmenu*		'wildmenu'  Always enabled since 9.0.0279
 T  *+windows*		more than one window; Always enabled since 8.0.1118.